I spoke at BIL Chennai last year about Agile. It was a great event and had
up to 1000 people in the audience across the day.
The Queen Mary once held up to 2000 passengers, but not sure, how many
people the ballrooms hold for the event. Should be more than e.g. the
average EclipseCon for sure;-)

BIL is more science oriented, and I guess in some cases, the "B" actually
stood for "Biology" when they started, so it's clearly more targeted towards
healthcare, pharma or general science, then TED or TEDx though it was
clearly inspired by those.

At the moment, there seems to be no BIL conference in Europe, but at least
interest by the TEDx organizers to speak with myself or Matthias and his
team raises hope, we could also get a speaking slot over here some day.